In a thermal type line printer generally an elongated print head is arranged transversely in relation to a sheet of printing paper for exerting heat on paper to produce characters of dots. The print head is constructed to move toward and away from a platen roll so that the beginning of the sheet of printing paper can easily pass through between the print head and the platen roll, prior to the printing step. The movement of the print head in relation to the platen roll is ordinarily performed by a cam mechanism. First the print head is set in a position further from the platen roll, and the end of the printing paper sheet is caused to pass through a space between the platen roll and the print head. Next, the print head is moved toward the platen roll until the print head presses the sheet against the platen roll with a preselected pressure. Printing is performed while the sheet is moved by rotating the roller. In addition, the exterior surface of the platen roll is covered with a rubber or the like layer to provide elasticity and to ensure forward movement of the printing sheet and the printing quality of characters on the sheet. Printing with the thermal printer is achieved by bringing a thermal head provided with numerous heating elements into contact with a thermally sensitive paper, and by using the heat from any of the heating elements on the sensitive paper to a color change and appearance of characters due to the heat.
Printing by thermal transfer printer is achieved by bringing a thermal head provided with heating elements into contact with a film having thermoplastic ink applied to its reverse side, and melting the ink with the heat from any of heating elements and then transferring the molten ink to normal paper.
In the prior art, as shown in the cam mechanism of FIG. 4, comprises a rotating cam 13 having a curved cam edge 13a. The radius (i.e. the distance from a rotating shaft 14) or the cam edge continuously increases away from the rotating shaft 14, and terminates in a straight line-like edge. The print head is supported on and guided along the curved edge 13a of the cam plate 13 so that with the rotation of the cam plate 13 the print head moves toward and away from the platen roll, depending upon the profile of the curved edge part 13a presenting points of different distances from the rotating shaft 14. Accordingly, when it is desired that the print head move away from the platen roll to set the end of the printing sheet along the platen roll, the cam plate 13 is rotated so that the print head is supported on a relatively small radius section on the side of the curved edge 13a of the cam. On the other hand, when it is desired that the print head is moved toward the platen roll and then press the sheet against the platen roll at a predetermined pressure for the printing step, the cam plate 13 is rotated at a predetermined angle of rotation by a limit switch or the like in such a direction that a section of relatively large radius side of the cam curved edge 13 approaches the print head, and then the cam 13 is stopped so that the print head becomes supported at a fixed position on the curved edge 13a which is mediately distant from the rotating shaft 14.
In operation of the above-described cam mechanism, however, it is very difficult to stop the rotating cam plate 13 accurately at a predetermined angle of rotation. Thus, with variation of rotational angle or the location at which the rotating cam 13 is stopped, the pushing force of the print head against the platen roll undergoes a change, so that a disadvantageous disturbance of printing characters is likely to occur on the paper.